I'm not sure what the attraction is for Zagreb. For us, it was a fine for a day spent before flying out- but unless you are into museums, not sure of the appeal. I sure would not spend time there - even if there was a Westin to stay on points. Plus the hotel location isn't anything special.
Re: Splt- we are working on plans for our return trip. We'll base ourselves at the Le Meridien and take day trips with our rental car - so one could spend 3-5 nights there easily.
I would not miss Dubrovnik or Plitvice Lakes.
You don't say how old you are- but I prefer Korcula to Hvar as Hvar has more of a bar/night scene which isn't our "thing". But an island stay is pretty special - and one can rent a great apartment for next to nothing.